So I had a quick look at the the slides this morning, and there seems no problem with fitting identical bearing sets to both. I have no idea why Arc said they weren't suitable. No doubt some unforeseen issue will raise it's head when I've machined some critical part!
Leadscrew ends:

Bearing housings:



Not sure what the three indents are on the inside of the topslide, but they're not modified anyway.
My intention is to mount each casting on the faceplate, aligned using a tight-fitting drill in the hole, then turn away the required amount of material. Then open up for the spindle bearing with a boring bar or appropriate sized drill.
I'm also fitting the Myford settable dials. KWIL very kindly sent me his drawings to modify the existing cast dials, but I'm a bit ashamed to say I'd rather spend the minimum possible time modifying machines. Having said that, £40 per dial is a bit steep, so if anyone knows of alternatives to Myford items I'd be interested in knowing.
Anyway, regarding the comment that it can't be done on the lathe that's being modified, I don't understand this. I can lock the topslide gibs, because I don't need it to slide, and use that casting temporarily on the cross slide. Machining a temporary housing or buying a replacement seems a waste of time and/or money; I propose to simply secure the existing one with the original caphead screws in conjunction with large washers and a couple of spacers like this:

After all I'm only going to use it once, and it seems to work fine. Can anyone see anything wrong with this method?
Cheers all.